1. Field of the Invention
The present invention relates to transmissions having an automated shift system.
2. Background Art
Transmissions for vehicles having an automated shift mechanism have been developed that automatically shift a shift lever mechanism similar to a manual transmission shift mechanism. One example of such a transmission has been developed for medium and heavy-duty trucks is known as the “AutoShift” transmission by Applicants' assignee primarily for medium and heavy-duty trucks. This system uses an electronic control that operates X-Y motors in a shift actuator to shift between a plurality of different gear trains to provide a range of gear ratios. Using this technique, operation of a vehicle is simplified and shifting performance may be optimized by reducing or minimizing human error. While the AutoShift system has proven effective in higher gear ratios, in lower gear ratios when the truck is operated at slow speeds, it would be desirable to provide quicker shift response when shifting from gear to gear.
In some transmission applications, it may be preferred to provide a wet clutch to disengage the transmission from the vehicle engine or source of drive torque to provide superior clutch durability. The torque load to the transmission is relieved by disengaging the clutch. Disengaging the clutch theoretically permits the torque load to go to zero and allows the transmission system to shift into neutral and prior to changing gears. However, with a wet clutch, even a small amount of rotation between the transmission and engine may cause the wet clutch to remain sufficiently engaged to prevent the transmission from being shifted into neutral. A wet clutch resists pulling to neutral as a result of “torque lock” caused by viscous drag in the wet clutch which may be as little as seven foot pounds of torque. The viscous drag is caused by the shearing of fluid between members that have a speed differential in the clutch pack.
If an X-Y shifter is provided, it may not be able to overcome the residual torque. If the X-Y shifter motors cannot overcome the residual torque, shifting will be delayed until the torque is reduced sufficiently to be overcome by the X-Y shifter motors. The time required to fully disengage the clutch may lead the operator to believe that the transmission is sticking or not properly shifting. A delay of a half a second or more may be noticeable to an operator.
There is a need for a control system and method of operating a vehicle transmission system that breaks, or reverses, the torque load resulting from wet clutch viscous drag. By counteracting the torque load from the wet clutch, one gear set can be disengaged allowing the transmission to be shifted into neutral. These and other problems facing prior art vehicle transmission systems are addressed by Applicants' invention as summarized below.